`TestSceneStatisticsPanel` intends to check the operation of statistics
panels using dummy ruleset classes. However, `StatisticsPanel` relies on
being able to retrieve the playable beatmap, which requires a converter.
One was not provided by the dummy rulesets, therefore the retrieval
would fail with an unobserved exception.
To fix, add a barebones converter implementation that is enough for the
test to pass.
The main goal here is to remove the inheritance, since realm doesn't
like that. Unfortunate that we can't use object initialisers in a few of
these places, but no real way around that.
